The present invention relates to an exhaust gas purification method.
In the past, in a diesel engine, particulate contained in the exhaust gas has been removed by arranging a particulate filter in the engine exhaust passage, using that particulate filter to trap the particulate in the exhaust gas, and igniting and burning the particulate trapped on the particulate filter to regenerate the particulate filter. The particulate trapped on the particulate filter, however, does not ignite unless the temperature becomes a high one of at least about 600xc2x0 C. As opposed to this, the temperature of the exhaust gas of a diesel engine is normally considerably lower than 600xc2x0 C. Therefore, it is difficult to use the heat of the exhaust gas to cause the particulate trapped on the particulate filter to ignite. To use the heat of the exhaust gas to cause the particulate trapped on the particulate filter to ignite, it is necessary to lower the ignition temperature of the particulate.
It has been known in the past, however, that the ignition temperature of particulate can be reduced if carrying a catalyst on the particulate filter. Therefore, known in the art are various particulate filters carrying catalysts for reducing the ignition temperature of the particulate.
For example, Japanese Examined Patent Publication (Kokoku) No. 7-106290 discloses a particulate filter comprising a particulate filter carrying a mixture of a platinum group metal and an alkali earth metal oxide. In this particulate filter, the particulate is ignited by a relatively low temperature of about 350xc2x0 C. to 400xc2x0 C., then is continuously burned.
In a diesel engine, when the load becomes high, the temperature of the exhaust gas reaches from 350xc2x0 C. to 400xc2x0 C., therefore with the above particulate filter, it would appear at first glance that the particulate could be made to ignite and burn by the heat of the exhaust gas when the engine load becomes high. In fact, however, even if the temperature of the exhaust gas reaches from 350xc2x0 C. to 400xc2x0 C., sometimes the particulate will not ignite. Further, even if the particulate ignites, only some of the particulate will burn and a large amount of the particulate will remain unburned.
That is, when the amount of the particulate contained in the exhaust gas is small, the amount of the particulate deposited on the particulate filter is small. At this time, if the temperature of the exhaust gas reaches from 350xc2x0 C. to 400xc2x0 C., the particulate on the particulate filter ignites and then is continuously burned.
If the amount of the particulate contained in the exhaust gas becomes larger, however, before the particulate deposited on the particulate filter completely burns, other particulate will deposit on that particulate. As a result, the particulate deposits in layers on the particulate filter. If the particulate deposits in layers on the particulate filter in this way, the part of the particulate easily contacting the oxygen will be burned, but the remaining particulate hard to contact the oxygen will not burn and therefore a large amount of particulate will remain unburned. Therefore, if the amount of particulate contained in the exhaust gas becomes larger, a large amount of particulate continues to deposit on the particulate filter.
On the other hand, if a large amount of particulate is deposited on the particulate filter, the deposited particulate gradually becomes harder to ignite and burn. It probably becomes harder to burn in this way because the carbon in the particulate changes to the hard-to-burn graphite etc. while depositing. In fact, if a large amount of particulate continues to deposit on the particulate filter, the deposited particulate will not ignite at a low temperature of 350xc2x0 C. to 400xc2x0 C. A high temperature of over 600xc2x0 C. is required for causing ignition of the deposited particulate. In a diesel engine, however, the temperature of the exhaust gas usually never becomes a high temperature of over 600xc2x0 C. Therefore, if a large amount of particulate continues to deposit on the particulate filter, it is difficult to cause ignition of the deposited particulate by the heat of the exhaust gas.
On the other hand, at this time, if it were possible to make the temperature of the exhaust gas a high temperature of over 600xc2x0 C., the deposited particulate would be ignited, but another problem would occur in this case. That is, in this case, if the deposited particulate were made to ignite, it would burn while generating a luminous flame. At this time, the temperature of the particulate filter would be maintained at over 800xc2x0 C. for a long time until the deposited particulate finished being burned. If the particulate filter is exposed to a high temperature of over 800xc2x0 C. for a long time in this way, however, the particulate filter will deteriorate quickly and therefore the problem will arise of the particulate filter having to be replaced with a new filter early.
Further, if the deposited particulate is burned, the ash will condense and form large masses. These masses of ash clog the fine holes of the particulate filter. The number of the clogged fine holes gradually increases along with the elapse of time and therefore the pressure loss of the flow of exhaust gas in the particulate filter gradually becomes larger. If the pressure loss of the flow of exhaust gas becomes larger, the output of the engine falls and therefore due to this as well a problem arises that the particulate filter has to be replaced quickly with a new filter.
If a large amount of particulate deposits once in layers in this way, various problems arise as explained above. Therefore, it is necessary to prevent a large amount of particulate from depositing in layers while considering the balance between the amount of particulate contained in the exhaust gas and the amount of particulate able to be burned on the particulate filter. With the particulate filter disclosed in the above publication, however, no consideration is given at all to the balance between the amount of particulate contained in the exhaust gas and the amount of particulate able to be burned on the particulate filter and therefore various problems arise as explained above.
Further, with the particulate filter disclosed in the above publication, if the temperature of the exhaust gas falls below 350xc2x0 C., the particulate will not ignite and therefore the particulate will deposit on the particulate filter. In this case, if the amount of deposition is small, when the temperature of the exhaust gas reaches from 350xc2x0 C. to 400xc2x0 C., the deposited particulate will be burned, but if a large amount of particulate deposits in layers, the deposited particulate will not ignite when the temperature of the exhaust gas reaches from 350xc2x0 C. to 400xc2x0 C. Even if it does ignite, part of the particulate will not burn, so will remain unburned.
In this case, if the temperature of the exhaust gas is raised before the large amount of particulate deposits in layers, it is possible to make the deposited particulate burn without leaving any, but with the particulate filter disclosed in the above publication, this is not considered at all. Therefore, when a large amount of particulate deposits in layers, so far as the temperature of the exhaust gas is not raised to over 600xc2x0 C., all of the deposited particulate cannot be made to burn.
An object of the present invention is to provide an exhaust gas purification method able to continuously remove by oxidation the particulate in exhaust gas on a particulate filter.
Another object of the present invention is to provide an exhaust gas purification method able to continuously remove by oxidation the particulate in exhaust gas on a particulate filter and simultaneously remove NOx in the exhaust gas.
According to the present invention, there is provided an exhaust gas purification method comprising carrying on a particulate filter for removing particulate in exhaust gas discharged from a combustion chamber an active oxygen release agent for taking in oxygen and holding oxygen when there is excess oxygen in surrounding and releasing the held oxygen in the form of active oxygen when the concentration of oxygen in the surroundings fall, maintaining an air-fuel ratio of the exhaust gas flowing into the particulate filter normally lean and occasionally switching it temporarily to rich to promote an oxidation reaction of the particulate on the particulate filter by the active oxygen released from the active oxygen release agent when the air-fuel ratio of the exhaust gas is switched to rich, and thereby remove by oxidation the particulate on the particulate filter without emitting a luminous flame.
Further, according to the present invention, there is provided an exhaust gas purification method carrying on a particulate filter for removing particulate in exhaust gas discharged from a combustion chamber an active oxygen release agent/NOx absorbent for taking in oxygen and holding oxygen when there is excess oxygen in surrounding and releasing the held oxygen in the form of active oxygen when the concentration of oxygen in the surroundings fall and for absorbing NOx in the exhaust gas when an air-fuel ratio of the exhaust gas flowing into the particulate filter is lean and releasing the absorbed NOx when the air-fuel ratio of the exhaust gas flowing into the particulate filter becomes the stoichiometric air-fuel ratio or rich, maintaining the air-fuel ratio of the exhaust gas flowing into the particulate filter normally lean and occasionally switching it temporarily to rich to promote an oxidation reaction of the particulate on the particulate filter by the active oxygen released from the active oxygen release agent/NOx absorbent and reduce the NOx released from the active oxygen release agent/NOx absorbent when the air-fuel ratio of the exhaust gas is switched to rich, and thereby removing by oxidation the particulate on the particulate filter without emitting a luminous flame, and simultaneously removing the NOx in the exhaust gas.